Material for aging-resistant ceramic vaporizers

ABSTRACT

A ceramic material is provided containing 45 to 55% by weight TiB 2  and 40.5 to 54.7% by weight BN and 0.1-1.5% by weight Ca in the form of a compound selected from the group consisting of the calcium oxides and calcium borates and mixtures thereof, which material contains from 0.2 to 3% by weight of a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.

BACKGROUND OF THE INVENTION

[0001] (1) Field Of The Invention

[0002] The invention relates to a material for aging-resistant ceramicvaporizers.

[0003] The most widely used process for coating flexible substrates withmetals, in particular with aluminum, is high-vacuum tape coating. In ametalization installation, the substrate to be coated is passed over acooled roller and, as it does so, is exposed to the aluminum vapor,which is deposited on the substrate surface as a thin metal layer.

[0004] To generate the constant vapor stream required, ceramicvaporizers are heated to about 1450° C. by direct passage of electriccurrent. Aluminum wire is continuously fed in, is liquefied on theceramic surface and is vaporized at a vacuum of approximately 10⁻⁴ mbar.

[0005] In metalization installations, a series of vaporizers arearranged in such a way that a uniformly thick aluminum layer isdeposited over the entire width of the tape. The vaporizers are held bycooled copper clamping jaws, both end-side and lateral clamping beingpossible.

[0006] The width and length of the vaporizer are selected according tothe vaporization rate required (g Al min⁻¹). The usual cross section ofvaporizers is rectangular. As well as vaporizers with a rectangularcross section, a range of other cross sections are also known. Detailsof these are to be found, for example, in U.S. Pat. No. 4,089,643 or DE19823908 (corresponds to the U.S. Application bearing Ser. No.09/315,320).

[0007] The resistance R of the vaporizer in the electric circuit can becalculated from the geometry of the vaporizer and the electricalproperties of the ceramic material (characterized by the resistivityR_(spec)) in accordance with equation (1).

R=R _(spec) ×L/A

[0008] L: Length of the vaporizer

[0009] A: conductive cross section

[0010] In the electric circuit, the vaporizer behaves as a metallicresistor, i.e. its resistance increases as the temperature rises.Therefore, it is additionally necessary to know the ratio betweenresistance at operating temperature (R_(HT)) and the resistance at roomtemperature (R_(RT)) in order to accurately characterize the electricalproperties. For ceramic vaporizers, the ratio R_(HT)/R_(RT) is between2.3 and 6.0.

[0011] A distinction is drawn between 2-component vaporizers(predominantly comprising TiB₂ as conductive phase and BN as insulator)and 3-component vaporizers (predominantly comprising TiB₂ as conductivephase and a mixture of BN and AlN as insulator), according to the numberof principal components in the composition of the vaporizers. In the3-component vaporizer, AlN replaces some of the BN for economic reasons,since AlN is considerably less expensive. Therefore, in general the3-component vaporizer contains 15-20% AlN. Such compositions aredescribed, inter alia, in patents (e.g. U.S. Pat. No. 3,915,900, column1, lines 10 and 11 and U.S. Pat. No. 4,089,643, column 2, line 5).

[0012] Since 2-component vaporizer materials have a lowerhigh-temperature bending strength than 3-component vaporizer materials,they generally also contain 0.1-5% by weight CaO.

[0013] After production, 2-component vaporizers are generally packagedin bags made from metalized film, in order to protect the vaporizersfrom taking up moisture during transport or storage prior to use. Thisis because it has been found that 2-component vaporizer material isdamaged in a characteristic way by the uptake of moisture. The damagebecomes apparent from a strongly rising resistance during heating of thevaporizer, which takes place by the direct passage of current inmetalization installations. In practice, this means that a vaporizermade from the damaged material cannot be heated in metalizationinstallations, since its resistance is too high for conventional powersupplies (10-15 volts secondary).

[0014] Despite this drawback, 2-component vaporizers are frequentlyused, since they can be operated at higher temperatures and cantherefore reach vaporisation rates which are up to approximately +50%higher than 3-component vaporizers. Furthermore, they have a higherservice life than 3-component vaporizers.

SUMMARY OF THE INVENTION

[0015] The object of the invention is to provide a ceramic materialwhich is suitable for the production of 2-component vaporizers and isbetter able to withstand atmospheric humidity and is therefore moreresistant to aging than known materials for 2-component vaporizers.

[0016] The object is achieved by a ceramic material containing 45 to 55%by weight TiB₂ and 40.5 to 54.7% by weight BN and 0.1-1.5% by weight Cain the form of a compound selected from the group consisting of thecalcium oxides and calcium berates and mixtures thereof, which materialcontains from 0.2 to 3% by weight of a compound selected from the groupconsisting of the oxides, carbides and nitrides of the elements Al, Si,Zr, Ti and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 depicts the resistance curves as a function of time with alinear increase in power before and after storage for a conventionalvaporizer.

[0018]FIG. 2 depicts the resistance curves as a function of time with alinear increase in power before and after storage for a vaporizer of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The ceramic material preferably contains 0.2 to 1.5% by weight ofa compound selected from the group consisting of the oxides, carbidesand nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.

[0020] The ceramic material particularly preferably contains 0.2 to 0.5%by weight of a compound selected from the group consisting of theoxides, carbides and nitrides of the elements Al, Si, Zr, Ti andmixtures thereof.

[0021] The compound selected from the group consisting of the oxides,carbides and nitrides of the elements Al, Si, Zr, Ti and mixturesthereof is preferably a compound selected from the group consisting ofthe nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.

[0022] The compound selected from the group consisting of the oxides,carbides and nitrides of the elements Al, Si, Zr, Ti and mixturesthereof is particularly preferably AlN.

[0023] Particularly preferably, the material according to the inventionconsists of the above mentioned components.

[0024] The vaporizer material according to the invention does not differfrom conventional 2-component vaporizer material in terms of itsproperties which are relevant for use (wetting with liquid aluminum,vaporisation rate at operating temperature, service life), but it doesnot present any time-dependent change in the materials properties. Inparticular, the resistivity does not change in the way which is knownfor 2-component vaporizer material.

[0025] The material according to the invention can be produced in theknown way for conventional 2-component vaporizer materials. By way ofexample, production can take place by hot-pressing of homogenous powdermixtures in graphite molds using ram pressure, temperatures of at least1800° C. being required in order to achieve solid bodies with sufficientdensity (U.S. Pat. No. 4,528,120, column 1, lines 36-47).

[0026] The compounds selected from the group consisting of the oxides,carbides and nitrides of the elements Al, Si, Zr, Ti and mixturesthereof are introduced in finely distributed form into a conventionalTiB₂/BN/CaO mixture. This preferably takes place as early as duringmixing of the components mentioned first.

[0027] TiB₂ and BN are preferably used in the form of fine powders, asis known in the prior art (for example U.S. Pat. No. 4,528,120, column5, table above Example 1).

[0028] The compounds selected from the group consisting of the oxides,carbides and nitrides of the elements Al, Si, Zr, pi and mixturesthereof are preferably used as powders with a mean particle size of from5 to 15 μm, preferably from 8 to 12 μm.

[0029] Such powders are commercially available, for example under thenames SiC Grade UF-10, TiC High Vacuum 250, ZrC Grade A, AlN Grade A andGrade B, Si₃N₄ Grade B7 from H. C. Starck (Goslar). They can also beproduced in a manner known per se from commercially available powders.For example, the AlN which is particularly preferably used can also beproduced by nitriding of aluminum powders and subsequent comminution in,for example, ball mills.

[0030] The material according to the invention is particularly suitablefor the production of vaporizers which are used to coat flexiblesubstrates with metals. The production of the vaporizers from a block ofthe material according to the invention takes place as is known in theprior art. This may, for example, be carried out by sawing out thedesired shape.

[0031] The invention thus also relates to vaporizers which consist ofthe ceramic material according to the invention.

[0032] The following example serves to explain the invention further:

EXAMPLES Example 1

[0033] Production and comparison of the resistance to aging of avaporizer according to the invention (EV) and a comparative 2-componentvaporizer (2K-VV) according to the prior art

[0034] Homogenous powder mixtures of the following composition wereproduced:

[0035] 2K-VV: 48% TiB₂, 51% BN, 1% CaO

[0036] EV: 48% TiB₂, 49.5% BN, 1% CaO, 1/5% AlN

[0037] These homogenous powder mixtures were each compressed by axial,two-sided hot pressing in graphite molds at temperatures of >1800° C.and a pressure of >25 MPa, to form sintered bodies with a densityof >95% of the theoretical density. In each case two vaporizers weremachined out of the hot-pressed sintered bodies by means of band saws.

[0038] The vaporizers were rectangular, with a vaporizer geometry of10×20×120 mm, which is customary for test purposes.

[0039] In each case one vaporizer according to the invention and oneconventional vaporizer were heated by a linear increase in the power toa level of 4 kW. This power is necessary in order to reach the workingtemperature which is typical for vaporizers with the selected vaporizergeometry and material composition.

[0040] The other two vaporizers were initially stored for 12 hours in adesiccator at 100% atmospheric humidity before being heated as describedabove.

[0041] The results are shown in FIGS. 1 and 2, which illustrate theresistance curve as a function of time with a linear increase in powerto a level of 4 kW.

[0042]FIG. 1 shows the results for a conventional vaporizer (VV).

[0043] Curve 1 shows the measured curve of the resistance for thevaporizer which is heated immediately. The increase in resistance whichis typical of undamaged material can be seen. The high-temperatureresistance which is established for the selected power of 4 kW isapprox. 2800 moc (μ Ohm cm).

[0044] Curve 2 shows the measured curve of the resistance for thematerial after prior storage for 12 hours in a desiccator under 100%atmospheric humidity at 20° C. It is clear that, as the power isincreased, the resistance rises rapidly, reaching levels of >10,000 moc.A vaporizer of this type will fail in a conventional tape coatinginstallation and has to be replaced.

[0045]FIG. 2 shows the results with the vaporizers according to theinvention (EV).

[0046] Curve 1 shows the measured curve of the resistance for thevaporizer which is heated immediately. The increase in resistance whichis typical of undamaged material can be seen. The high-temperatureresistance which is established for the selected cower of 4 kW isapprox. (μ Ohm cm).

[0047] Curve 2 shows the measured curve of the resistance for thematerial after prior storage for 12 hours in a desiccator under 100%atmospheric humidity at 20° C. It is clear that after storage there isno damage corresponding to that which can be seen in FIG. 1.

[0048] Further tests (results not shown) have demonstrated that evenextending the storage time to a few days does not cause any damage. Theeffect described is even achieved with lower additive levels of0.3-0.5%.

[0049] Examples for vaporizer material containing ≦0.5% AlN. Thevaporizers EV 2 to EV 6 were produced in a similar manner to thatdescribed in Example 1. High-temp. High-temp. AlN TiB₂ BN CaO resistanceresistance Sin- content content content content before after tered [% by[% by [% by [% by storage storage body weight] weight] weight] weight][moc] [moc] EV 2 0.4 47.7 50.9 1.0 2725 2730 EV 3 0.3 48.7 50.0 1.0 24702480 EV 4 0.4 47.2 51.4 1.0 2830 2840 EV 5 0.5 46.4 52.1 1.0 3380 3300EV 6 0.3 48.5 50.2 1.0 2290 2300

Example 2

[0050] Vaporisation rate and service life of a vaporizer according tothe invention compared to known vaporizers

[0051] A vaporizer according to the invention (EV1) and a commerciallyavailable 3-component vaporizer (3K-VV) of the same dimensions(commercially available under the name ESK-Trimet® from ESK, Kempten)and the 2-component vaporizer in accordance with Example 1 (2K-VV) wereoperated in the test metalization installation under standard conditions(P<10⁻³ mbar; end-side clamping) using the power specified in Table 1.The measured vaporisation rate and service life are given in Table 1.Power Vaporisation [KW] rate [g/mincm²] Service life [h] EV 4 0.35 102K-VV 4 0.35 10 3K-VV 3.2 0.25  9 ← economic operation 4 0.35  6 2.4 0.210

1. A ceramic material containing 45 to 55% by weight TiB₂ and 40.5 to54.7% by weight BN and 0.1-1.5% by weight Ca in the form of a compoundselected from the group consisting of the calcium oxides and calciumborates and mixtures thereof, which material contains from 0.2 to 3% byweight of a compound selected from the group consisting of the oxides,carbides and nitrides of the elements Al, Si, Zr, Ti and mixturesthereof.
 2. The ceramic material as claimed in claim 1, which containsfrom 0.2 to 1.5% by weight of a compound selected from the groupconsisting of the oxides, carbides and nitrides of the elements Al, Si,Zr, Ti and mixtures thereof.
 3. The material as claimed in claim 1,which contains from 0.2 to 0.5% by weight of a compound selected fromthe group consisting of the oxides, carbides and nitrides of theelements Al, Si, Zr, Ti and mixtures thereof.
 4. The material as claimedin one of claims 1 to 3, wherein the compound selected from the groupconsisting of the oxides, carbides and nitrides of the elements Al, Si,Zr, Ti and mixtures thereof is selected from the group consisting of thenitrides of the elements Al, Si, Zr, Ti and mixtures thereof.
 5. Thematerial as claimed in claim 4, wherein the compound selected from thegroup consisting of the nitrides of the elements Al, Si, Zr, Ti andmixtures thereof is AlN.
 6. A process for producing the material asclaimed in one of claims 1 to 5, wherein a compound selected from thegroup consisting of the oxides, carbides and nitrides of the elementsAl, Si, Zr, Ti and mixtures thereof is introduced in finely distributedform into a TiB₂/BN/CaO mixture which is conventional for 2-componentvaporizers, and the homogenous powder mixture is hot-pressed in agraphite mold using ram pressure, at temperatures of at least 1800° C.7. The process as claimed in claim 6, wherein the compound selected fromthe group consisting of the oxides, carbides and nitrides of theelements Al, Si, Zr, Ti and mixtures thereof is used as a powder with amean particle size of from 5 to 15 μm.
 8. A vaporizer which consists ofmaterial as claimed in one of claims 1 to 5.